A stable electrical discharge is an important element desirable in the discharge chamber of a high power gas laser. It is known, for example, that the highest power operation in a gas laser system is generally limited by the discharge instability which manifests itself as a collapse from a distributed glow to a constricted arc within the discharge area. This collapse results in termination of laser discharge. Many methods have been used in the past in an attempt to improve the discharge stability and to overcome the glow breakdown.
In a typical high power gas laser operation, as much as 97% of the power input to the laser system may be dissipated as heat. Unless this heat is removed from the system, the increasing energy level of unexcited molecules caused by thermal energy is so great in the lower energy level that lasing action will eventually cease. It is desirable, therefore, to have relatively high speed gas flow within the discharge chamber for effective heat removal. High speed gas flow also helps to constrict an arc formation that may occur within the discharge chamber.
Pre-ionization is required in a laser discharge chamber in order to properly obtain the necessary uniform discharge. Applying direct current (D.C.) on the electrode is not sufficient to initiate and maintain a stable discharge. To obtain the necessary pre-ionization, various techniques have been utilized. Some of the methods used are photo-ionization, electron-beam and pulse-enhanced pre-ionization. The aforementioned photo-ionization methods are generally restricted to Excimer laser systems. In a high-power CO.sub.2 laser system, both electron-beam sustained and pulse-enhanced pre-ionization methods are exclusively used. In an electron-beam sustained discharge system, there is a need to separate lasing chamber from electron gun which produces electron beam. Since electron gun must be separated from lasing chamber because of the operating pressure difference, the separation is mechanically difficult. Because of these difficulties, pulse-enhanced pre-ionization methods are used for high power gas laser systems. Initial uniform pre-ionization in the discharge chamber creates a downstream instability because of the difference in ionic density between upstream and downstream side of discharge chamber. The difference in ionic density results from the additional ionization of gas molecules by the sustainer power during lasing operation.
In many systems, particularly transversely excited high power gas lasers, while the number of molecules available for ionization at the upstream end of the discharge chamber is relatively satisfactory, the flow of the gas through the chamber which is required for a continuous discharge gives rise to a non-uniform distribution of ions in the discharge chamber with a substantially greater number of ions at the downstream end. This increases the propensity for the discharge to break down with the result that arcing occurs between the electrodes at the downstream end of the chamber.
Yet a further problem in previous lasers has been to cool the electrodes. Electrodes generate a large amount of heat during discharge which must be dissipated not only to reduce the energy level of the unexcited molecules in the discharge chamber but also to reduce the wear of the electrodes and their arcing tendency. Many cooling techniques have been used such as circulating coolant around and within the discharge electrodes. These techniques, however, have been less than successful with the result that, as time goes by, electrodes continue to degenerate, arcing and wear again become problems.
Utilization of a forced-vortex flow and its application to a gaseous lasing medium has been described by Zerr in U.S. Pat. No. 4,612,646 dated Sept. 16, 1986. The Zerr system has a tangential gas inlet at one end of the chamber and a gas diffuser at the other end so that a forced-vortex will be established within the discharge chamber. In this system, however, the tangential velocity of gas flow is gradually changed as the gas travels towards the exit of the discharge chamber, resulting in non-uniformity within the discharge chamber which results in the concomitant arcing and discharge breakdown.